1. Field of the Invention
The present invention pertains to a catheter that inserts into a patient's esophagus to measure physiological characteristics of the patient, and, in particular, to a monitoring catheter that, once inserted, measures the activity of the patient's diaphragm and/or measures pressures within the patient at multiple locations. The present invention also pertains to a method of using such a catheter to measure these physiological characteristics.
2. Description of the Related Art
There are many instances where it is important to determine a patient's diaphragmatic function and/or inspiratory effort. For example, it is not uncommon for a patient in an intensive care unit ("ICU") to have a breathing disorder that causes the patient to go into ventilatory failure and/or to have difficulty weaning from a mechanical ventilator. It is typically difficult to predict clinical deterioration or diagnose the cause of such a breathing disorder to optimize the settings on the ventilatory assist device or to determine a propitious time to initiate attempts to wean the patient from the mechanical ventilator. For example, the physician must determine whether the patient is failing to spontaneously ventilate properly due to muscle weakness or whether failure to properly ventilate is due to a dysfunctional central nervous system. If, for example, the patient's failure to properly ventilate is due to muscle weakness, the physician can then focus on determining the cause of the weakness, e.g., whether the weakness is the result of malnutrition, myopathy, drug effects, etc. Furthermore, from time to time, a patient may not ventilate adequately despite being treated with mechanical ventilation. This may have one or more of a number of causes. For example, there may be intrinsic positive end exhalation pressure (PEEP) or other patient-ventilator disharmony which, if identified, may be addressed.
Conventional patient monitoring devices are incapable of easily measuring the physiological conditions of the patient needed to access the diaphragmatic function and/or inspiratory effort of the patient. For example, the conventional technique for measuring esophageal pressure is to provide a catheter in the patient's esophagus. One conventional catheter has a plurality of pressure ports at its distal end with a balloon surrounding the ports. The balloon is inflated with about 0.5 cc of air to prevent blockage of the ports. The pressure ports are connected to a common lumen so that the catheter measures the pressure at one location within the patient. Such a device approximates the intrapleural pressure by detecting respiratory pressure swings while minimizing the detection of pressure caused by cardiac activity.
This conventional device is disadvantageous in that it only measures the pressure at one location in the patient, for example, at the distal end of the catheter. Measuring multiple pressures requires multiple pressure measuring catheters. In addition, conventional pressure measurement catheters require a separate insertion procedure, apart from the procedure required to insert a feeding tube in the patient. This separate insertion procedure exposes the patient to the additional risk that the conventional pressure sensor will be unintentionally inserted into the trachea tube.
It is also known that one way to measure diaphragm muscle activity using an electromyogram ("EMG") is to place electrodes on the surface of the patient's abdomen proximate to the diaphragm. Because the major portion of the diaphgram muscle is deep within the patient's abdomen, these external EMG electrodes are often not well suited to detect diaphragm EMG signals or are contaminated by signals from the muscles overlying the abdomen, as well as the cardiac activity of the patient. An article by Onal et al., entitled "Effects of Electrode Position on Esophageal Diaphragmatic EMG in Humans," published in volume 47, no. 9, pages 1234-8 of the Journal of Applied Physiology in December 1979 teaches that it is known to use an esphogeal catheter with multible electrodes disposed on an exterior surface to measure EMG activity. However, a suitable monitoring device using such internal EMG measurment techniques has not gained popular acceptance.